1. Field of the Invention--The present invention relates to enhanced error correction capabilities by testing for all zeros in the prefix and completion frames of high density magnetic tapes and, in particular, the present invention relates to an enhanced correction capability for testing prefix and completion frames for all zeros in IBM 3480 high density magnetic tape before entering the adaptive cross-parity (AXP) correction process.
2. Discussion of Prior Art--The IBM 3480 magnetic tape sub-system is a high-density eighteen-track, parallel, magnetic tape storage sub-system. The IBM 3480 magnetic tape forms the basis of the "Proposed American National Standard Magnetic Tape and Cartridge for Information Exchange Eighteen-Track Parallel, 12.65 MM (1/2 inch), 1491 CPMM (37 871 CPI) Group-Recording as prepared by Technical Committee X3B5 and as set forth in X3B5/88-044, Feb. 9, 1988 published by the American National Standard Institute.
The IBM 3480 high density tape incorporates an error correction system utilizing adaptive cross-parity (AXP) code. This is discussed in Patel, "Adaptive Cross-Parity AXP Code for a High Density Magnetic Tape Sub-System", IBM Journal Research and Development, Vol. 29, No. 6, Nov. 1985, Pgs. 546-562 and in U.S. Pat. No. 4,201,976.
In the proposed ANS Standard and as shown in FIG. 1, the prefix frames are all zeros and the completion frames in tracks B1 through B7 and A1 through A7 are all zeros. In FIG. 1, there are two sets of tracks: set A and set B. Set A has nine tracks, A0 through A8, and set B has nine tracks, B0 through B8. In the example shown in FIG. 1, there are six frames shown. When the tape moves in the forward direction, as shown by arrow 10, the frames are read in the order of FC1 through FC6. When the tape is read in the reverse direction as shown by arrow 20, the frames are renumbered and are also read in the order of FC1 through FC6. When the tape is read in the forward direction 10, frames FC1 and FC2 are the prefix frames. When the tape is read in the reverse direction 20, frames FC1 and FC2 are the completion frames. The prefix frames are all zeros and the completion frames in tracks Bl through B7 and Al through A7 are all zeros unless an error occurs. Frames FC3 and FC4 comprise the channel supplied or customer data which varies in length and content. Tracks A8 and B8 are the VRC tracks which provide the vertical redundancy check. Tracks A0 and B0 are the DRC tracks which provide the diagonal redundancy check. The use of the VRC and DRC tracks are fully discussed in the Patel paper.